It is well known that the voltage output of conventional magnetic reading heads is dependent upon both the magnetic field intensity incident upon the head and the rate of change of the field intensity per unit time. This of course implies that when attempting to read the magnetic field intensity associated with or emanating from an article or other material, the voltage output from the head is dependent upon the speed of movement of the article or material relative to the head.
Perhaps the most conventional approach to nullifying the effect of the speed dependency is to design precise mechanical systems for moving the article or material at a constant speed relative to the head. This approach of course is ubiquitous in the audio and high fidelity industry. Unfortunately such precise mechanical systems are expensive, highly subject to malfunction and unduly restrictive of the speed at which a measurement of field can be made.
In technical fields such as those relating to computers and credit cards where the magnetic information to be sensed is in the form of relatively regularly spaced patterns of digital signals, there have been numerous relatively complex electronic systems, many involving closely spaced dual magnetic heads, adapted for using the regularity and periodicity of the information as a way of achieving relative independence from the speed problem. However, such techniques are not seen to be adaptable to reading analog or non-digital information.
Another approach to achieving relative independence from the speed problem has been the use of Hall-effect readout heads which are essentially independent of this rate of change of the magnetic field incident thereupon. See, for example, "Hall-Effect Magnetic Sensor Reads Data at any Speed", Electronics, Feb. 1, 1973. However, Hall-effect magnetic sensors are relatively expensive, not readily adaptable to measuring areas less than about 0.010 inches by 0.020 inches, and typically suffer from temperature instability problems. Consequently, some of the advantages provided by the use of Hall-effect readout heads are essentially nullified in that relatively complex and expensive temperature stabilizing and/or compensating networks typically must be used in connection therewith.
Still another approach to achieving relative freedom from the speed dependency problem was the invention of the semiconductive magnetic transducer, as described in U.S. Pat. No. 3,389,230, and as further described in EDN, Feb. 15, 1969, at pages 73-78. This semiconductive magnetic transducer, commonly called the magnistor, is essentially a dual collector bipolar transistor which relies on diversion of current from one collector to the other in the presence of a magnetic field as an indication of the intensity of the magnetic field. The magnistor is relatively free from the speed dependency problem; however its relative fragility and unreliable availability after more than ten years since its announcement to the public have not satiated the long felt need for a simple and economical magnetic reading apparatus and method which is independent of the speed of movement of an article or material relative thereto.